The present invention relates to a method for producing synthetic resin films with a predetermined contour and/or a uniform or nonuniform thickness distribution by a separation and/or ablation machining of preformed synthetic resin films, especially for the production of flexible leaflets (e.g. cusps, flaps] for artificial heart valves.
Thin plastic films can be produced by various processes as, for example, injection molding, deep drawing, dip molding or casting. To the extent that the films can have a thickness of 500 xcexcm and more, and the thickness distribution over the entire foil area is not significant or has only a subordinated significance, as a rule no after treatment of the foil is required over the area thereof and the foil can be stamped to produce the desired contour. It is different, however, when for such synthetic resin foils, a reproducible adjustable uniform thickness or thickness variations are desired in a locally defined manner which can be in a range of 30 xcexcm to 500 xcexcm.
Prosthetic heart valves are comprised of a support housing with a base ring carrying at least two posts extending substantially in the axial direction of the ring and which are connected by an arcuate wall structure serving for fastening of the flexible leaflets or cusps as has been described, for example, in DE 38 34 545 C2. The three leaflets there shown for an aortic heart valve insure the closing and opening, whereby the leaflets at their free edges lie against one another with formation of an overlapping region in the closed state. The known construction and that described in DE 42 22 610 A1 tend to closely resemble the natural aortic valve which has the connection line of the leaflet or cusp with the natural aortic root approximately formed by the intersection of a cylinder with the aorta. At this region, the connecting line closes the upstream side of the commissure, at which the lines or the leaflets contact. The commissures prevent a breakdown of the leaflets and serve in combination with the leaflet overlap regions to brace the leaflets. From the foregoing, it is apparent that the leaflets connected with the stent, whether they form a heart valve with three or two leaflets are differently stressed at different locations, whereby depending upon the type and magnitude of the mechanical stress, the respective thicknesses should be locally adjusted. Apart from regions of greater thickness, there thus must be regions of thicknesses below 50 xcexcm.
Apart from the desired thickness distributions, it is advantageous to obtain the smoothest possible and most homogeneous-free leaflet edge. Both the optimum thickness distribution and also the smooth leaflet edge influence to a significant degree of durability of the prosthesis since the leaflets must be capable of withstanding billions of bending load alternations. Injection and dip casting processes are indeed basically used to produce a foil with local differences in thickness although these processes cannot provide the desired degree of reproducibility of the dimensions and also cannot provide fabrication thicknesses below 100 xcexcm. Such thicknesses can only be obtained with extremely high apparatus costs. The quality of the free leaflet edges is insufficient when the cusps are made by these processes.
It is thus the object of the present invention to provide a process of the initially described type which permits a foil to be obtained with extremely small thicknesses and with locally different thickness distributions as well as with a smooth closure edge of the cut foil piece.
These objects are achieved with a method which is characterized in accordance with the invention in that the synthetic resin foil is separated by means of a laser beam bundle along the predetermined contour and/or is subjected to ablation in an area-wise manner until the respective desired thickness dimension is obtained.
Surprisingly, it has been found that the area-wise ablation and separation can be carried out reproducibly with the requisite precision while the synthetic resin surface is locally melted at respective laser burn spots and the melted material vaporized. The differences in the separation and area-wise ablation by means of a laser lies only in the penetration depth or machining depth, whereby the separation or cutout of the desired synthetic resin foil contour is effected by forming a groove with growing depth. The laser and the optics associated therewith form a machining tool which does not come into contact with the workpiece so that no wear is generated. Via corresponding servomotors, exact guidance of the laser beam bundle is possible. The process of the invention thereby has the advantage that, from the viewpoint of the synthetic resin foil thickness dimension as produced by injection, dip casting or spraying, there are no requirements from the point of view of the desired thickness distribution since the foil can be directly subjected to a material removal process. This means in addition that initially there is a significant simplification in the process technology since thicker foils can be produced which are later correspondingly profiled and contoured.